1. Field of the Invention
The present invention relates to a system for setting up paths for implementing a media information distribution service, such as a video-on-demand service, using a broadband exchange technology such as a broadband-integrated services digital network (B-ISDN).
2. Description of the Related Art
A two-way broadband service, such as a video-on-demand system which enables subscribers to watch video on demand, is now being implemented using a B-ISDN broadband exchange technology.
FIG. 1 shows a general configuration of such a two-way broadband service system.
In a central office 701, a video server 702 which provides video services is connected to an ATM (Asynchronous Transfer Mode) exchange 703 by a line 707. To the ATM exchange 703 is connected by a line 708 a distributor 704, which concentrates subscribers' lines 709.
Digital image data output, placed in ATM cells, from the video server 702 is transmitted to set top boxes (STB) 705 via line 707, ATM exchange 703, line 708, distributor 704, and subscribers' lines 709 in the order mentioned.
The STB 705 converts digital image data into a television signal and then supplies it to a television monitor (TV) 706 to display video.
On the other hand, control information on a video program selection made by a subscriber is transferred from the STB 705 to the distributor 704 over the subscribers's line 709. Then, the distributor 704 transfers the control information to the video server 702 via an ethernet 710. Next, the video server 702 responds to the control information to send digital image data corresponding to the video program the subscriber desires to that subscriber. Response data is transferred from the video server 702 to the STB 705 via the ethernet 710, distributor 704, and subscribers's line 709.
At this point, as a path for communicating image information to a subscriber, a virtual path must be set up between the ATM exchange 703 and the video server 702 and between the ATM exchange 703 and the distributor 704 using a VPI (Virtual Path Identifier) and a VCI (Virtual Channel Identifier) defined in ATM cell headers.
Path setup systems include an SVC (Signaling Virtual Channel) system in which a path is set up at a subscribers's request and a PVC (Permanent Virtual Channel) system in which a path is set up on a fixed basis at provisioning time (at the start of service).
In the SVC system, at the setup of a path in the ATM exchange, control information is exchanged among the ATM exchange 703, the video server 702, and the distributor 704 as requested by a subscriber, thereby permitting a path to be set up between the ATM exchange 703 and the video server 702 and between the ATM exchange 703 and the distributor 704. If, therefore, a number of requests for video-on-demand service is made simultaneously by subscribers, then control information traffic and equipment transactions will increase, resulting in poor response times.
In the PVC system, on the other hand, a PVC for video-on-demand service is established between the ATM exchange 703 and the video server 702 by allocating a predetermined VPI to the line 707 between the ATM exchange 703 and the video server 702, and a PVC for video-on-demand service is also established between the ATM exchange 703 and the distributor 704 by allocating a predetermined VPI to the line 708 between the ATM exchange 703 and the distributor 704. The video server 702 appends that predetermined VPI allocated to the line 707 and a VCI by each subscriber to ATM cells and then sends them onto the line 707. The ATM exchange 703 replaces the predetermined VPI allocated to the line 707 in the ATM cells with the predetermined VPI allocated to the line 708 and then sends the ATM cells over the line 708. The distributor 704 receives ATM cells to which the predetermined VPI allocated to the line 708 is appended and then sends the ATM cells over a subscribers's line 709 corresponding to the VCI appended to the ATM cells.
In the above-described PVC system, ATM cell processing units 801, installed in the video server 702 shown in FIG. 2 to convert digital image data read from a disk storage unit 802 into ATM cells and send them over the line 707, have a limit on their number that is connectable to one line 707 because of limited line capacity. In order to provide video-on-demand service to many subscribers, therefore, it is required to install a plurality of lines 707 (in FIG. 2 two lines: #1 and #2) and connect n number of ATM processors 801 and one disk storage unit 802 to each of the lines. On the other hand, a plurality of distributors 704 may also be needed to accommodate many subscribers, in which case the number of lines 708 required also increases with the number of distributors 704. In addition, as shown in FIG. 2, a plurality of lines 708 may be connected to each of the distributors 704 (in FIG. 2 two lines: #1 and #2, or #3 and #4).
On this assumption, in the prior art, the distributor 704 accommodates a set of subscriber groups and each subscriber group is allocated a set of PVC (VPI) on the line 708 and a PVC (VPI) on the line 707. That is, the distributor 704 allocates only ATM cells received over the line 708 and appended with a predetermined VPI to a subscriber group and only ATM cells received over the line 708 and appended with another predetermined VPI to another subscriber group. Note that each subscriber in each subscriber group is identified by a VCI appended to each ATM cell.
For example, in the prior art, VPI=10 on the line 707 (#1) connected to the video server 702 is connected to VPI=0 on the line 708 (#1) connected to the distributor 704 (#1) by the ATM exchange 703 as shown in FIG. 3. The distributor 704 (#1) allocates only VPI=0 on the line 708 (#1) to a subscriber group comprised of subscribers #0 to #23 for example.
Likewise, VPI=11 on the line 707 (#1) connected to the video server 702 is connected to VPI=1 on the line 708 (#2) connected to the distributor 704 (#1) by the ATM exchange 703. The distributor 704 (#1) allocates only VPI=1 on the line 708 (#2) to a subscriber group comprised of subscribers #24 to #47 for example.
On the other hand, VPI=20 on the line 707 (#2) connected to the video server 702 is connected to VPI=0 on the line 708 (#3) connected to the distributor 704 (#2) by the ATM exchange 703. The distributor 704 (#2) allocates only VPI=0 on the line 708 (#3) to a subscriber group comprised of subscribers #0 to #23 for example.
Likewise, VPI=21 on the line 707 (#2) connected to the video server 702 is connected to VPI=1 on the line 708 (#4) connected to the distributor 704 (#2) by the ATM exchange 703. The distributor 704 (#2) allocates only VPI=1 on the line 708 (#4) to a subscriber group comprised of subscribers #24 to #47 for example.
The possibility of simultaneous requests for video-on-demand service by all the subscribers is considered to be very low. It is thus uneconomical to install in the video server 702 as many ATM cell processing units 801 as there are subscribers. In general, the number of the ATM cell processing units 801 connected to the two lines 707 is set, within the limit determined by the line capacity, equal to the number obtained by multiplying the total number of subscribers allocated to the two lines 707 by the percentage obtained by adding the average percentage of subscribers who may simultaneously make requests for video-on-demand service and the deviation from the average.
As an example, assumes that, in the example of FIG. 3, the total number of subscribers allocated to the two lines 707 is 96, the average of the percentage of subscribers who simultaneously make requests for video-on-demand service is 50%, and the deviation from the average is 10%. Then, the number of ATM cell processing units 801 required for the two lines 707 will be EQU 96.times.(50%+10%).apprxeq.58
Statistically it is known that the smaller the total number of subscribers allocated to one line 707 is, the greater the deviation from the average (in the above example, 10%) becomes. In the prior art described previously, the total number of subscribers that are allocated to one line 707 is 48.
If, therefore, all 96 subscribers could be allocated to one line 707, then the deviation from the average would be, say, about 5%. In this case, the number of ATM cell processing units 801 required would be EQU 96.times.(50%+5%).apprxeq.53
Thus, the number of ATM cell processing units 801 required in the entire video server 702 in the case where all 96 subscribers are allocated to two lines 707 becomes larger than in the case where it is assumed that all 96 subscribers can be allocated to one line 707. That is, the prior art suffers from a problem that, when the capacity of one line 707 is small and the number of lines 707 connected to the video server 702 has to be increased accordingly, the number of ATM cell processors 801 required in the video server will also increase.